1. Field of the Invention
The present invention generally relates to a fuel cell system. More specifically, the present invention relates to a method and apparatus used for determining a humidity state in a fuel cell.
2. Background Information
A fuel cell is an electrochemical device that employs an electrochemical reaction between hydrogen and oxygen to generate electric energy or power. In the fuel system, output from the cell decreases due to an insufficient supply of gas, poor power generation caused by drops of liquid resident in a channel, and reduced electric conductivity caused by dryness in an electrolyte. When the fuel cell is operated under such conditions, a problem arises in that deterioration of the cell is accelerated. Therefore, the humidity state in the fuel cell must be determined in order to resolve the abovementioned problem.
In Japanese Laid-Open Patent Application No. 2004-127915, a method is disclosed for determining the humidity state. In this publication, excessive humidity is presumed when an average cell voltage is within a first predetermined range and a cell block exists where the output voltage is outside a second predetermined range. Insufficient humidity is presumed when the average cell voltage is outside the first predetermined range and voltage variance is within a third predetermined range. An insufficiency of fuel gas is presumed when the voltage variance is outside the third predetermined range.
In Japanese Laid-Open Patent Application No. 2002-184438, a technique is disclosed wherein the position in a stack occupied by a unit cell whose voltage has decreased is determined. If the unit cell is positioned near an end, then the stack is determined to be in an excessively humid state, whereas if the unit cell is positioned near the middle, the stack is determined to be in an insufficiently humid state.
In Japanese Laid-Open Patent Application No. 9-245826, a method is disclosed wherein the humidity state is determined by storing beforehand in a storage device a pattern of changes in voltage over time for a cell block that comprises one or more unit cells, measuring the pattern of changes in voltage over time for each unit cell or cell block, and comparing the resulting voltage pattern and the stored pattern.
In Japanese Laid-Open Patent Application No. 2005-228688, voltages are measured for each of a plurality of unit cell. If the minimum unit cell voltage exceeds a first predetermined value when a factor influencing the load or the wetted state changes, the fuel cell is determined to be wet, and the result of the determination takes effect if the voltage falls below a second predetermined value if the factor affecting wetness is still changing or has finished changing.